It is known to provide hydrogen tanks for fuel-celled vehicles. In these vehicles, steels of the types 18/10-Cr/Ni or 18/12-Cr/Ni, for example 1.4404, 1.4435 or 1.4571, are used for hydrogen storage and supply components. These steels are meta-stable steels, even though it requires a rather severe cooling and deformation to cause a martensite change. Because of the increased addition of Ni, these steels are more expensive than those of the type 18/8-Cr/Ni. Nitrogen is not a typical alloy element in these types of steels. Currently, these steels are used due to the existence of hydrogen embrittlement. However, due to the meta-stability of the material, brittleness may still exist.
The phenomenon of hydrogen embrittlement of a material, in particular steel, is well known in the art. The hydrogen penetrates the structure of the material and compromises its integrity. The hydrogen reduces the material's mechanical qualities, in particular its ductility such as elongation at fracture (A) or Reduction of Area (Z). Depending on the structure, some steels are very sensitive to hydrogen embrittlement. A number of studies have shown that the sensitivity to hydrogen embrittlement is lower with the cubic face centered (fcc) austenitic structure than the cubic body centered (bcc) ferritic/martensitic structure.
Austenitic steels can be divided into stable austenitic steels and meta-stable austenitic steels. The stable austenite, the austenitic structure, is not altered, regardless of how cold the working temperature is and/or how large the deformation. The cause of this stability is the large portion of austenitic alloy elements, in particular, nickel, manganese, nitrogen, and to a smaller degree carbon (to 2%). A typical representative of this steel is DIN1.4439. The carbon content is usually limited to about 0.03 wt %.
The meta-stable austenite is partially converted to martensite by cooling and/or deformation of the material. Typical representative types of steel are those of type 18/8-Cr/Ni, for example, DIN1.4301/AISI304. The carbon content is usually limited to about 0.07 wt % due to the formation of chrome carbides during manufacturing of the steel. On the other hand, carbon stabilizes the austenitic structure.
Nitrogen is not a typical alloy element for these kinds of steels, but nitrogen stabilizes the austenitic structure when incorporated in a certain amount. It is further known that, when the material is exposed in a hydrogen atmosphere, any damage to the material with tend to cause a tear (fracture) to propagate at the surface of the material.
The most common materials used for hydrogen applications are stainless steel because of their low susceptibility to environmental hydrogen embrittlement (HEE). Stainless steel can be divided into stable and meta-stable grades. Since at meta-stable grades (typically those of types 18Cr-8Ni) parts of the structure undergo a transformation from face centered cubic (fcc) austenite to body centered cubic (bcc) α′ martensite when cold formed and/or cooled down to very low temperatures, the structure of stable austenitic steels (typically those of types 18Cr-12Ni) remains austenitic independent of the operating or work hardening conditions.
For stationary hydrogen tanks where cost and weight are of minor importance, grade Cr18-Ni10 steels of types 1.4404 (AISI 316L) or 1.4571 (316 Ti) are widely and successfully used. Usually, wall thicknesses are quite high which results in a low failure probability. Nickel is the cost driver in stainless steel, which makes these grades unattractive for automotive vehicle applications where cost and weight are of major importance. Unfortunately meta-stable grades like DIN 1.4301 (AISI 304) suffer from severe HEE whereas the influence of hydrogen on grade AISI 316L is slight or negligible. It is known that the fcc austenitic structure is quite insensitive to HEE and that the severe HEE of meta-stable grades can be attributed to the γ-α′-transformation.
The main phenomena of HEE are shown in FIG. 1. Hydrogen enters the material via adsorption and dissociation of the H2 molecule followed by absorption of the H proton, while the electron is released into the free electron gas of the metal. The H protons diffuse into areas of high tensile stresses where they accumulate and embrittle the material. The most plausible theories are the “decohesion theory” and the “HELP theory”. While the atomistic processes of hydrogen embrittlement are not quite understood yet, it is common sense that hydrogen enters the metallic structure via the above-described surface or near surface processes (adsorption, dissociation, absorption, and diffusion). One precondition for these processes to take place is the destruction of the oxide layer due to local plastic strain. The heat released by local plastic deformation provides enough energy so that adsorption, dissociation, and absorption can take place easily on the newly formed (not oxidized) metal surfaces.
Thus, it is desirable to stabilize the austenitic structure of the steel. Ni, Mn, C, and N are the elements that stabilize the austenitic structure, of which C and N are the most inexpensive ones. It is also desirable to incorporate compressive stresses that counteract with external tensile stresses. It is further desirable to reduce or suppress diffusivity of hydrogen in the lattice. It is still further desirable to control surface processes (adsorption, dissociation, absorption, and diffusion) so that the hydrogen cannot enter the lattice. It is yet further desirable to use specific gas impurities like oxygen for a spontaneous reformation of the oxide layer, which inhibits the entire process. Therefore, there is a need in the art to treat austenitic steel that meets at least one of these desires.